The present invention relates to a photocatalyst which can clean the environment by decomposing environmental contaminants, etc., and more particularly to a technical field of a photocatalyst which causes a conspicuous improvement in photocatalytic efficiency in quantum-mechanical terms by supporting nanoscale ultra-fine metal particles, which are capable of realizing a quantum size effect, on fine particles of a photocatalyst, and a highly functional base material which can clean the environment with high efficiency by holding the photocatalyst supporting ultra-fine metal particles on the surface of such a material.
A photocatalytic reaction of titanium dioxide was reported in Nature in 1972 and became known world-wide as the Honda-Fujishima effect. Since then, research has been conducted on the production of hydrogen and carbon dioxide by the decomposition of water or the decomposition of aqueous solutions of organic matter by means of titanium dioxide under irradiation with light; and today, a technique in which fine particles of titanium dioxide are held in the form of a thin film on tiles or window glass is in the process of being adapted for practical use in the decomposition of environmental contaminants, i.e., tobacco tar and organic matter such as bacteria, toxins produced by bacteria, etc.
Titanium dioxide is a powder-form metal oxide and is used after being dispersed in a solution in the case of decomposition of water or:.solutions. However, it is desirable that titanium dioxide adhere to window glass and bathroom tiles, or to the surfaces of construction materials, in the form of a uniform thin film even when titanium dioxide is used in particle form. Accordingly, methods such as sol-gel methods, spray pyrolysis methods using titanium acetate, etc., and dip coating methods, etc. have been developed. These techniques are described in xe2x80x9cOyo Butsuri (Applied Physics)xe2x80x9d, Vol. 64, No. 8, p. 803 (1995), xe2x80x9cKayak to Kogyo (Chemistry and Industry)xe2x80x9d, Vol. 48, No. 10, p. 1256 (1995), and xe2x80x9cKayak to Kogyo (Chemistry and Industry)xe2x80x9d, Vol. 49, No. 6, p. 764 (1996). It has been shown that adhering oils and tobacco tar can be decomposed while being irradiated with ultraviolet light using a glass, etc. covered with such titanium dioxide. It is difficult to decompose inorganic matter such as dirt and dust, etc.; however, it has been reported that since such inorganic matter adheres with organic substances such as oils, etc. as a binder, the decomposition of organic matter also tends to prevent the adhesion of inorganic matter.
The principle of the action of fine particles of titanium dioxide on materials such as tiles, etc. is based on the photocatalytic characteristics of titanium dioxide as a semiconductor. If titanium dioxide is irradiated with light which exceeds the band gap energy, e. g., ultraviolet light, then the electrons in the valence electron band are excited and undergo a transition to the conduction band, so that positively charged holes are left in the valence electron band, thus producing electron-hole pairs. These electrons and holes move through the titanium dioxide and reach the surface; and the electrons are supplied to oxygen in the air so that O2xe2x88x92 (super-oxide anion) is produced and other substances are reduced. The holes not only cause direct oxidative decomposition of organic matter but also oxidize water molecules adhering to the surface so that strongly oxidizing hydroxyl radicals are formed, and other substances are oxidized by the oxidizing power of these hydroxyl radicals. The O2xe2x88x92 reportedly participates in this oxidation process; however, the detailed reaction circuit is still being studied. Thus, organic matter is decomposed into carbon dioxide and water by electron-hole pairs excited by light.
In this research, instances have been found in which the electrons and holes re-couple and disappear prior to the oxidation-reduction of external substances in cases where titanium oxide is used alone. Accordingly, it has been indicated that there are limits to the photocatalytic efficiency of titanium dioxide. The ordinary state of titanium dioxide is a powdered state; considering a single particle of titanium dioxide, there are countless lattice defects such as point defects and plane defects, etc. in the surface and interior portions of the particle. When electrons and holes excited in titanium dioxide by ultraviolet light encounter lattice defects in the process of movement, these electrons and holes are captured by the lattice defects and caused to re-couple. In some cases, furthermore, even if the electrons and holes are able to move to the surface, the electrons and holes re-couple when they approach each other. In order to ameliorate such problems, it is necessary to develop techniques for manufacturing titanium dioxide which is free of lattice defects, and techniques for separating electrons and holes at the surface. In regard to the former techniques, improvements have been made in crystal growth techniques; however, since these techniques have no direct connection with the present invention, a detailed description will be omitted here.
In regard to techniques for separating electrons and holes at the surface, a photocatalyst has been proposed in which an electrode which collects excited electrons is formed on the surface of titanium dioxide, so that holes are separated and collected on the surface of the titanium dioxide, while electrons are separated and collected on the surface of the metal electrode. If this approach is used, electrons can be efficiently collected on the surface of the metal electrode, and holes and electrons can be separated; accordingly, the probability of re-coupling would appear to be lowered. Photocatalysts of this type are referred to as xe2x80x9cmetal-supporting photocatalystsxe2x80x9d, and are manufactured by forming metals conventionally used as catalysts, such as Pt (platinum) and Cu (copper), etc. on the surface of titanium dioxide. The idea here is that if the metals have a catalytic effect even when used alone, then a synergistic effect with the catalytic action of titanium dioxide should be exhibited.
Methods that have been developed for manufacturing such metal-supporting photocatalysts include: photo-deposition methods in which the semiconductor is suspended in an aqueous solution of a metal salt, after which a reducing agent is added and irradiation with light is performed; impregnation methods in which the semiconductor is immersed in an aqueous solution of a metal salt and dried, after which a reduction treatment is performed; chemical deposition methods in which the semiconductor is violently agitated in an aqueous solution of a metal salt and a reducing agent is added; and simultaneous precipitation methods in which an aqueous solution of a metal salt is added to the semiconductor raw material and simultaneous precipitation is performed, after which sintering is performed. Furthermore, other methods which have been developed include: kneading methods in which the semiconductor and a powdered metal are mixed in a mortar, shaking mixing methods in which the semiconductor and a powered metal are placed in a vessel and mixed by shaking using a shaker, etc.; and powdered metal addition methods in which the semiconductor and a powdered metal are separately added to reaction product solutions and are then suspended and mixed.
The present inventors successively investigated these methods but were unable to form micron-sized fine metal particles that have a particle diameter of 0.1 microns or greater on the surface of titanium dioxide. In other words, the inventors reached the conclusion that the formation of nano-scale ultra-fine metal particles capable of exhibiting a quantum size effect on the surface of a semiconductor is difficult as long as powdered metals or aqueous solutions of metal salts are used. Furthermore, in such conventional methods, the number of fine metal particles that can be supported on one titanium dioxide particle (i.e., the supported particle density) is limited to the range of a few tens of particles. The reasons for such a small supported particle density are that there are problems in the manufacturing methods used, and at the same time, since the particle diameter of the fine metal particle is large, not many fine metal particles adhere to the surface of one titanium dioxide particle. Thus, according to measurements performed by the present inventors, the photocatalytic efficiency of metal-supporting photocatalysts produced by conventional manufacturing methods shows only about a two- to four-fold reinforcement compared to the photocatalytic efficiency of titanium dioxide used alone.
The present inventors made a theoretical investigation, with reference to FIG. 18, concerning the problem of why the photocatalytic efficiency is not greatly reinforced by micron-sized fine metal particles.
In order to achieve efficient incorporation of the electrons generated in titanium dioxide into a metal electrode, it is desirable that the electron transition barrier at the interface between the titanium dioxide and the metal be as small as possible. However, in cases where the particle diameter of the fine metal particles is micron-sized (approximately 0.1 microns or greater), the electron state has roughly the same band structure as a large solid crystal (bulk crystal). In other words, a structure is obtained in which the valence electron band and the conduction band are distinctly formed, with the bands being separated by a fixed band gap, and with free electrons being densely packed in order from the bottom to the uppermost Fermi level in the conduction band. Meanwhile, since titanium oxide is a bulk crystal, the electron state naturally adopts a band structure. In such a band structure, the energy levels constituting the bands are densely arranged more or less continuously, and wave functions corresponding to the respective levels are sharply localized within the substance. In other words, since the wave functions do not extend beyond the substance, the probability that the electrons residing at these levels will be released to the outside of the substance is fairly small.
When, in this state, titanium dioxide is irradiated with ultraviolet light so that electrons are excited into the conduction band so as to cause electron-hole pairs to be formed, in order for these electrons to reduce an external substance A and produce super-oxide anions, it is necessary that the electrons quickly move from the titanium dioxide into the metal, and that the electrons further move from the metal into external substance A outside the metal. However, as described above, since the fine metal particles are micron-sized particles, not only does the electron state adopt a band structure similar to that of a large crystal, but the wave functions also have a structure which is sharply localized within the fine metal particles. Accordingly, since it is not easy for the electrons which have climbed to the conduction band of the titanium dioxide to enter the wave functions of the metal, it is likewise not easy for the electrons to move to the conduction band of the metal. Furthermore, even if the electrons somehow manage to move into the metal, it is similarly difficult for the electrons to move from the metal into an external substance; and in most cases, therefore, before the electrons can leave the metal, the electrons quickly drop to the Fermi level EF which is in the conduction band of the metal, so that the chance of reacting with the external substance is further diminished.
More specifically, in cases where the level density of the conduction band is high as in the band structure of a bulk crystal, the time required for electrons to drop to the Fermi level (i.e., the relaxation time) is extremely short; and this, together with the localization of the wave functions, prevents the movement of electrons to the outside. In other words, in the case of micron-sized particles, since it is difficult for electrons to move to the outside, electrons are accumulated to an excessive degree inside the metal, so that the movement of electrons from the titanium dioxide into the metal is conversely prevented by the repelling electric field. In the final analysis, it may be concluded that in cases where the particle diameter of the fine metal particles is in the micron size region, electrons remain inside the titanium dioxide or fine metal particles as a result of the energy band structure and localization of the wave functions, so that the probability of electrons to be released to the outside of the metal is reduced to a small value. At the same time, in the case of micron-sized fine metal particles, the number of fine metal particles that can be supported on one titanium dioxide particle is limited to several tens of particles; these are the reasons why the photocatalytic efficiency of such metal-supporting photocatalysts is limited.
Photocatalysts have a decomposing effect on environmental contaminants; and an idea of endowing such photocatalysts with an adsorbing effect has also appeared. Substances that provide such an adsorbing effect include porous materials such as active carbon, active carbon fibers and zeolite, etc. The active carbon fiber is shown in FIG. 19, and in the surface of this fiber, countless pores with a diameter of approximately 0.5 nm, i.e., so-called xe2x80x9cmicroporesxe2x80x9d, are opened. Environmental contaminants such as organic substances, etc. are adsorbed in these micropores. Since the active carbon fibers can be worked into a variety of shapes, they are widely used in water cleaners and air cleaners.
If such active carbon fibers are used as a base material and caused to hold a photocatalyst, then it should be possible for the active carbon fibers to adsorb environmental contaminants so that the photocatalyst can decompose these environmental contaminants. A deodorizing device in which a photocatalyst is held on active carbon is described in Japanese Patent No. 2574840. FIG. 20 is a conceptual diagram of a photocatalyst in which anatase type titanium dioxide is held on active carbon fibers. If all of the organic matter adsorbed in the micropores were to be decomposed by the photocatalyst, then an adsorbing/decomposing power with an efficiency of 100% would be obtained. However, as described above, since there are limits to the decomposing power of titanium dioxide used alone, some organic matter remains in the micropores. Consequently, the adsorbing power of the active carbon fibers gradually drops until at some point only the decomposing power of the anatase type titanium dioxide remains. Thus, it has been found that the initially expected effect cannot be obtained. The main reason for this is the above-described limit on the photocatalytic efficiency of anatase type titanium dioxide, while a break-through improvement in photocatalytic efficiency is hoped for.
Titanium dioxide includes anatase type titanium dioxide and rutile type titanium dioxide according to differences in crystal structure. Of these two types, rutile type titanium dioxide has a more stable structure. When heated to approximately 600xc2x0 C. or higher, all anatase type titanium dioxide undergoes a phase transition to a rutile type, and this rutile type remains at low temperatures following cooling. Even at temperatures below 600xc2x0 C., a portion of such anatase type titanium dioxide is converted into rutile type titanium dioxide. Accordingly, rutile type titanium dioxide can be mass-produced more inexpensively than anatase type titanium dioxide. Conventionally, however, all of the titanium dioxide used as a photocatalyst has been anatase type titanium dioxide, and inexpensive rutile type titanium dioxide has not been used at all. The reason for this may be understood from the band structure.
The band structure of rutile type titanium dioxide is shown in FIG. 21. The gap energy thereof is 3.05 eV. Electrons excited to the conduction band by ultraviolet light reach the bottom of the conduction band while expending a portion of their energy due to relaxation. Since the oxygen potential which is the reduction potential is positioned at 3.13 eV, the climbing of electrons from the bottom of the conduction band to the oxygen potential requires external energy and tends not to happen spontaneously. Accordingly, in the case of rutile type titanium dioxide, it is difficult to form super-oxide anions.
Meanwhile, FIG. 22 shows the band structure of anatase type titanium dioxide. Here, the gap energy thereof is 3.20 eV, so that even if the excited electrons drop to the bottom of the conduction band following excitation by ultraviolet light, oxygen at 3.13 eV can be sufficiently reduced. Thus, anatase type titanium dioxide has the capacity to produce super-oxide anions. Accordingly, in conventional techniques, it has been necessary to use expensive anatase type titanium dioxide as a photocatalyst.
Accordingly, the first object of the present invention is to find a method that makes it possible to achieve a great improvement in the photocatalytic efficiency of titanium dioxide. The second object of the present invention is to find a method which makes it possible for rutile type titanium dioxide, which has not been utilizable as a photocatalyst in the past despite its low cost, to be utilized as a photocatalyst. The third object of the present invention is to provide an effective means of cleaning the environment by causing fine particles of a reinforcing photocatalyst to be held on various types of base materials. The fourth object of the present invention is to realize a photocatalyst which causes an effective continuation of the adsorbing power of active carbon fibers, etc., thus achieving an extended useful life of the adsorption/decomposition cycle and especially to realize a practically effective means of cleaning the environment.
The photocatalyst supporting ultra-fine metal particles that is provided by the present invention is constructed by supporting ultra-fine metal particles with a particle diameter that makes it possible to manifest a quantum size effect on fine particles of a photocatalyst. As a result of the quantum size effect of the ultra-fine metal particles, photo-excited electrons are forcibly expelled from the metal, so that super-oxide anions can be produced with a high efficiency, thus allowing high-speed decomposition of environmental contaminants.
By setting the mean particle diameter of the ultra-fine metal particles in the range of 1 to 10 nm, the quantum size effect can be maximally heightened.
By setting the supported particle density of the metal, i.e., the mean number of ultra-fine metal particles supported on one fine photocatalyst particle, at 100 ultra-fine particles or greater, the efficiency of environmental contaminant decomposition can be increased, and a light source consisting of natural sunlight can also be used.
By constituting the ultra-fine metal particles with a transition metal, and by constituting photocatalytic substance with a metal oxide semiconductor which has the capacity to produce hydroxyl radicals and/or super-oxide anions when irradiated with ultraviolet light, a photocatalyst supporting ultra-fine metal particles which has a high reactivity can be realized.
By selecting the ultra-fine metal particles from Pt, Au, Pd, Rh or Ag, and the fine photocatalyst particles are made of titanium dioxide, then a photocatalyst supporting ultra-fine metal particles which has a high degree of stability and safety can be realized.
Even if rutile type titanium dioxide is used, a photocatalytic efficiency which is comparable to that of anatase type titanium dioxide can be obtained, and the photocatalyst supporting ultra-fine metal particles can be mass-produced at low cost.
By holding such a photocatalyst supporting ultra-fine metal particles on a base material, it is possible to obtain a highly functional base material in which a high photo-degrading power is added to the intrinsic functions of the base material.
Since molding that uses fibers as a base material can be executed extremely easily, fibers are widely used in a variety of devices such as deodorizers, water cleaners and air conditioners, etc.
If the fibers used are fibers of an adsorbing material, then environmental contaminants can be adsorbed on these fibers of an adsorbing material and decomposed by the photocatalyst supporting ultra-fine metal particles. Accordingly, a long-term adsorbing/decomposing function can be obtained merely by installing the photocatalyst.
By setting the weight of the photocatalyst supporting ultra-fine metal particles to be 1% or more of the weight of the above-described fibers of an adsorbing material, an adsorption-decomposition equilibrium can be realized.
By molding such highly functional fibers, it is possible to obtain various types of highly functional fiber products such as fabrics, curtains and carpets, etc. which have a high photo-degrading power.
By mixing an organo-metallic compound that can be reduced by heating and fine photocatalyst particles in a solvent so that the organo-metallic compound is caused to adhere to the surfaces of the fine photocatalyst particles, and then removing the solvent from this mixed solution and firing the residue, it is possible to support numerous nano-scale ultra-fine metal particles on the surfaces of the fine photocatalyst particles.
By spraying fine photocatalyst particles and a solution of an organo-metallic compound that can be reduced by heating toward each other so that numerous particles of the organo-metallic compound are caused to adhere to the surfaces of the fine photocatalyst particles, and by firing the fine photocatalyst particles while they are falling downward, a photocatalyst supporting ultra-fine metal particles can be continuously manufactured.
With a use of a hydrophobic colloid of an organo-metallic complex as the above-described organo-metallic compound, a photocatalyst supporting nano-scale ultra-fine metal particles can be reliably manufactured.
By allowing the photocatalyst supporting ultra-fine metal particles to be electrostatically adsorbed on the surface of the base material, a highly functional base material can easily be obtained.
A highly functional base material with a long useful life which holds a photocatalyst supporting ultra-fine metal particles can be obtained by means of a continuous process that comprises a first step in which a colloid of an organo-metallic compound is caused to adhere to fine photocatalyst particles, a second step in which the fine photocatalyst particles to which the colloid is adhering are applied to a base material, and a third step in which this base material is fired so that the organo-metallic compound is reduced, thus causing the ultra-fine metal particles to be firmly supported on the fine photocatalyst particles, and at the same time causing these fine photocatalyst particles to be firmly held on the base material.
Furthermore, a highly functional base material with a long useful life which holds a photocatalyst supporting ultra-fine metal particles can be obtained by means of a continuous process that comprises a first step in which a colloid of an organo-metallic compound is caused to adhere to fine photocatalyst particles, a second step in which the fine photocatalyst particles to which the above-described colloid is adhering are fired so that the organo-metallic compound is reduced, thus causing the ultra-fine metal particles to be firmly supported on the fine-photocatalyst particles, and a third step in which these photocatalyst particles supporting ultra-fine metal particles are applied to a base material and fired so that the fine photocatalyst particles are firmly held on the base material.
By modifying a fiber raw material into active carbon fibers via a carbonization treatment and an activation treatment, and by using the resulting active carbon fibers as a base material so as to support a photocatalyst supporting ultra-fine metal particles by one of the manufacturing methods described above, a method for the continuous manufacture of highly functional active carbon fibers from fiber raw materials (the fibers themselves or fabrics, etc.) can be obtained.
By mixing fine photocatalyst particles supporting ultra-fine metal particles with a substance which has the property of spreading into a thin film on a solvent, and after forming this substance into a thin film by spread it on a solvent and then by causing the thin film to be held on the surface of a two-dimensional base material or three-dimensional base material, a highly functional base material with a planar shape or three-dimensional shape can be obtained.
The present invention will be described in detail below.
The present inventors conducted diligent research in an attempt to strengthen the photocatalytic function of titanium dioxide supporting a metal. As a result of this research, the inventors discovered that the photocatalytic function can be increased approximately 100 times or more compared to titanium dioxide used alone by supporting nano-scale ultra-fine metal particles on the surface of titanium dioxide. Thus, even compared to titanium dioxide supporting micron-scale fine metal particles, an approximately 3- to 25-time increase in photocatalytic efficiency is possible. This can be achieved by converting the metal from fine particles into ultra-fine particles, i.e., by converting the particle diameter from the micron scale to the nano-scale, or in other words, by decreasing the particle size to a value that is approximately {fraction (1/10)} to {fraction (1/100)} of the micron scale (approximately 0.1 microns or greater). The mean particle diameter of the ultra-fine metal particles used in the present invention is 1 to 10 nm, preferably 1 to 5 nm. If the mean particle diameter is set at a larger value, the manifestation of the quantum size effect that will be described later is diminished, so that the strengthening of the photocatalytic efficiency is no longer conspicuous. On the other hand, if the mean particle diameter is set at a smaller value, the particle size approaches atomic size; as a result, the manufacture of the ultra-fine metal particles becomes technically difficult, and at the same time becomes extremely expensive.
In cases where a fine particle-form powder of titanium dioxide, etc. is utilized as a photocatalytic substance, the number of ultra-fine metal particles that can be supported on one fine photocatalyst particle, i.e. the supported particle density of the ultra-fine metal particles, is an important element along with the particle diameter. In the present invention, the use of ultra-fine metal particles which have been greatly reduced in size to the nano-scale makes it possible to support numerous ultra-fine metal particles on a single photocatalyst particle. More specifically, as a result of realizing an extreme reduction in the particle diameter from fine metal particles on the micron scale to ultra-fine metal particles on the nano-scale, a dramatic improvement in the supported particle density is achieved. According to research performed by the present inventors, it is desirable that the mean number of ultra-fine metal particles supported on one fine photocatalyst particle be set at 100 particles or greater, and preferably at 200 particles or greater. If the supported particle density is 100 particles or greater, the photocatalytic efficiency can be conspicuously increased compared to conventional photocatalysts as a result of a synergistic effect with the quantum size effect. If this number is 200 particles or greater, a great improvement in the photocatalytic efficiency can be achieved. Of course, if the supported particle density can be increased even further, an even greater increase in the photocatalytic density can be achieved.
The quantum size effect, which is manifested only by ultra-fine metal particles, will be discussed below. For example, considering an ultra-fine particle with a diameter of 1 nm, the number of metal atoms present in such a particle is only about 10 to 100 atoms depending on the size of the atoms. Furthermore, an ultra-fine metal particle with a diameter of 10 nm would be presumed to contain approximately 10,000 to 100,000 atoms. In the case of ultra-fine metal particles which thus contain a small number of atoms, the electron energy state of the metal gradually begins to become discrete from the band structure, so that the energy levels are distributed across a broad range. For example, considering the conduction band, the numerous energy levels constituting the conduction band are distributed from a tightly packed state upward and downward across a broad range while being separated from each other. This discrete separation of the energy levels has the effect of lengthening the relaxation time of the electrons, i.e. the time required for the electrons to drop from these levels to the Fermi level. In other words, the time that electrons are resident at a given level is extended. At the same time, this has the effect of causing the wave functions corresponding to the energy levels to protrude to the outside of the metal while extending shoulders to the left and right, and simultaneously causing the peaks of the wave functions to be lowered. In other words, the electrons carried on these wave functions can easily move to the outside as a result of the quantum tunneling effect. The term xe2x80x9cquantum tunneling effectxe2x80x9d used in the present invention refers to the manifestation of a quantum tunneling effect caused by the discrete separation of the energy levels and the non-localization of the wave functions as described above.
FIG. 1 shows a state in which a photocatalyst supporting ultra-fine metal particles is held on active carbon fibers. It is desirable that the weight of the photocatalyst supporting ultra-fine metal particles be equal to 1% or more, and preferably 3% or more, of the weight of the fibers. If this weight ratio is less than 1%, the effect of the photocatalyst supporting ultra-fine metal particles becomes insufficient. The photocatalyst consists of rutile type titanium dioxide, and ultra-fine metal particles with a particle diameter of several nm are supported on this rutile type titanium dioxide at a high density. First, the active carbon fibers adsorb organic matter consisting of environmental contaminants, and this organic matter is packed into the micropores of the active carbon fibers. Next, the photocatalyst supporting ultra-fine metal particles begins to break down the organic matter.
So as to examine how titanium dioxide supporting ultra-fine metal particles performs an efficient oxidation-reduction reaction with respect to organic matter, FIG. 2 shows the energy state in a case where ultra-fine metal particles are supported on titanium dioxide. When the titanium dioxide is irradiated with ultraviolet light, electron-hole pairs are formed, and the electrons are excited to the conduction band, leaving the holes in the valence electron band. In the case of excitation by ultraviolet light with a large energy, the electrons make a transition to a high potential in the conduction band; however, the electrons drop to the bottom of the conduction band while gradually losing energy. Since the energy levels of the metal are separated at a certain density, an energy level that corresponds to the bottom of the conduction band of the titanium dioxide always exists. Furthermore, the wave function of this level extends long shoulders to the left and right, so that the left end of the wave function extends into the titanium dioxide, and the right end of the wave function extends to the outside of the metal. In other words, the energy levels of the titanium dioxide and the metal are continuous (in resonant terms) via the wave functions of the metal. Excited electrons in the conduction band of the titanium dioxide are carried on the wave functions of the metal and are promptly expelled to the outside via the metal as a result of the quantum tunneling effect. Since the titanium dioxide and the metal are in a resonant state, this quantum tunneling effect is referred to as xe2x80x9cresonance tunnelingxe2x80x9d. In this case, since the levels in the metal are discretely separated, the relaxation time is lengthened, so that the electrons can easily be released to the outside of the metal before dropping to the Fermi level of the metal.
The holes present in the valence electron band of the titanium dioxide move to the surface of the titanium dioxide and oxidize the external substance D. Furthermore, not only are external substances oxidized, but also water adhering to the surface is thought to be oxidized so that strongly oxidizing hydroxyl radicals are produced, and it is thought that these hydroxyl radicals also cause the oxidative decomposition of external substances. Meanwhile, the electrons released to the outside of the metal by resonance tunneling not only cause direct reduction of the external substance A but also reduce oxygen in the air, thus producing super-oxide anions O2xe2x88x92; and it is thought that these anions also participate in the decomposition of the external substance D.
In particular, the excited electrons which move from the titanium dioxide into the metal in the present invention do not accumulate in the metal, but are instead immediately released to the outside, so that no repelling electric field is formed on the outside. Accordingly, a superior reducing power is obtained in that the excited electrons produced by irradiation with ultraviolet light can be successively attracted.
The type of photocatalyst used in the present invention is not limited to titanium dioxide but is rather determined by the substance that is to be decomposed, i.e. the substance that is to be subjected to oxidation-reduction. In cases where the substance that is to be decomposed is a substance that is reduced, a reduction potential exists; and in the case of a substance that is oxidized, an oxidation potential exists. It is necessary that the above-described reduction potential and oxidation potential be positioned within the energy gap between the valence electron band and the conduction band of the photocatalytic substance. To describe this in greater detail, it is desirable to select a photocatalytic substance which is such that the reduction potential is positioned on the upper side within the gap, and the oxidation potential is positioned on the lower side within the gap, as shown in FIG. 2. In this case, excited electrons can drop from the bottom of the conduction band to the reduction potential and reduce the target substance, and holes can climb from the upper end of the valence electron band to the oxidation potential and oxidize the target substance. However, since resonance tunneling of the ultra-fine metal particles also has an effect in the present invention, the reduction potential may also be positioned at the bottom of the conduction band or slightly higher.
Since rutile type titanium dioxide is used in FIG. 2, the reduction potential is positioned 0.08 eV above the bottom of the conduction band as in FIG. 20. In this case, the excited electrons are carried on the wave function of the metal at this position by resonance tunneling before the electrons drop to the bottom of the conduction band, so that external substances are quickly reduced as a result of resonance tunneling from the metal level. The fact that rutile type titanium dioxide can be utilized as a photocatalyst in the same manner as anatase type titanium dioxide as a result of this resonance tunneling is of very great significance. As a result of the present invention, inexpensive rutile type titanium dioxide which can be mass-produced has attracted attention as a photocatalyst for the first time. As a result of the present invention, photocatalysts may quickly become popular in a wide range of applications.
Furthermore, in research performed in recent years, it has been considered that the electrons reduce O2 to produce super-oxide anions O2xe2x88x92; the holes oxidize water to produce hydroxyl radicals, and these O2xe2x88x92 anions and hydroxyl radicals decompose the target substances. Accordingly, photocatalytic substances can also be selected with the O2 potential selected as the reduction potential, and the OH potential selected as the oxidation potential. In other words, a photocatalytic substance which may be used is such that electron-hole pairs are produced by irradiation with ultraviolet light, oxygen in the air or in water is reduced by the electrons so that super-oxide anions are produced, and water adhering to the surface is oxidized by the holes so that hydroxyl radicals are produced.
A semiconductor is appropriate as the photocatalytic substance used. In the case of insulators, the gap energy is excessively large so that the production of electron-hole pairs by means of ordinary ultraviolet light is difficult. On the other hand, in the case of substances which have a small gap energy, it is difficult to position the oxidation and reduction potentials within the forbidden band, and such substances tend to dissolve in aqueous solution; accordingly, substances of this type are unsuitable.
Among semiconductors, metal oxide semiconductors are especially appropriate for use in the present invention. Compared to simple metals, metal oxides are extremely stable substances; thus, such substances have a low reactivity with other substances and are therefore safe. Moreover, metal oxides are sufficiently capable of electron transfer. Accordingly, metal oxide semiconductors which have such properties can be utilized as the photocatalytic substance of the present invention. For example, such metal oxides can be selected in accordance with the substance that is the object of decomposition from presently known substances including WO3, CdO3, In2O3, Ag2O, MnO2, Cu2O3, Fe2O3, V2O5, TiO2, ZrO2, RuO2, Cr2O3, CoO3, NiO, SnO2, CeO2, Nb2O3, KTaO3, SrTiO3 and K4NbO17, etc. Among these metal oxides, TiO2, SrTiO3 and K4NbO17 are especially desirable from the standpoints of the density of electron-hole pairs produced, the density of super-oxide anions and hydroxyl radicals produced and material properties such as corrosion resistance and safety, etc.; and TiO2, i.e., titanium dioxide, is the most desirable.
Photocatalytic substances which can be used in the present invention consist of fine particles. Since fine particles have an extremely large surface area, the probability of contact with environmental contaminants is large; at the same time, numerous ultra-fine metal particles can be supported on the surfaces of such fine particles. Furthermore, fine particles have a larger effective light-receiving surface for ultraviolet light, etc., so that the photocatalytic efficiency is greatly improved compared to that of bulk substances. Since metal oxides ordinarily consist of powders, a metal oxide semiconductor such as titanium dioxide is suitable for use in the present invention. The particle diameter used is 30 nm to 1,000 nm, preferably 50 nm to 500 nm. If the particle diameter is smaller than this, the particles approach the size of ultra-fine particles; as a result, special techniques are required for manufacture, and in addition, the cost increases. On the other hand, if the particle diameter is greater than this, the specific surface area is decreased so that the reactivity with environmental contaminants, substances which are toxic to the human body and substances with unpleasant odors, etc. deteriorates.
For example, titanium dioxide can be formed into ultra-fine particles with a diameter of approximately 10 nm; however, such particles do not remain in the state of independent particles, but instead form aggregations of ultra-fine titanium dioxide particles so that large titanium dioxide clumps of the type described above are formed after all. In this case, since the particles are stuck together in an uneven configuration, the surface area is larger than in the case of single large particle, so that the reactivity is increased. The present invention includes such fine photocatalyst particles also. There are no particular restrictions on the shape of the fine photocatalyst particles as long as these particles are capable of supporting ultra-fine metal particles; for example, shapes of spherical, pellet-form or granule-form particles, etc. may be employed.
The light source used in the present invention may be any light source which has an energy greater than the band gap energy of the photocatalyst. Ordinarily, an ultraviolet lamp is used. Especially in cases where titanium dioxide is used, both rutile type and anatase type titanium dioxide may be encountered; and if the respective gap energies of these two types of titanium dioxide are converted into wavelengths, the value for rutile type titanium dioxide is 407 nm, and the value for anatase type titanium dioxide is 388 nm. Accordingly, it is desirable that the wavelength distribution of the light source used for titanium dioxide have a peak in the vicinity of 400 nm. A moth-attracting lamp with the wavelength distribution shown in FIG. 3 has a peak in the vicinity of 400 nm; accordingly, such a lamp can be used for both rutile type and anatase type titanium dioxide and is therefore extremely desirable.
Natural sunlight with the wavelength distribution shown in FIG. 4 is mostly a visible light but includes 400 nm; therefore, such light is adequate for use. In the case of natural sunlight, the light intensity is higher at 407 nm than at 388 nm; accordingly, rutile type titanium dioxide is more effective than anatase type titanium dioxide in this case. Thus, the fact that rutile type titanium dioxide can be utilized as a photocatalyst in the present invention opens an important path allowing the use of natural sunlight. This contrasts with the case of conventional anatase type titanium dioxide, which, while allowing the use of ultraviolet lamps, shows an extremely low catalytic efficiency in the case of natural sunlight. Furthermore, in the case of conventional photocatalysts, the use of outdoor sunlight is possible, since the light intensity is strong; however, in the case of indoor use, the light intensity is weak, so that such use is a weak point of conventional photocatalysts. In the present invention, on the other hand, since the photocatalytic efficiency is greatly improved, the indoor utilization of photocatalysts using natural sunlight as a light source can be expanded.
It is desirable that the ultra-fine metal particles that are supported be particles of a transition metal. Transition metal elements are elements which have incomplete d shells, and these elements are metallic elements comprising four groups with atomic numbers ranging respectively from 21 (Sc) to 29 (Cu), 39 (Y) to 47 (Ag), 57 (La) to 79 (Au), and 89 (Ac) to a theoretical 111. Since the d shells of these elements are incomplete, the outermost shells possess directionality according to the d electrons; as a result, excited electrons coming from the photocatalytic substance can easily be captured at the surfaces of the ultra-fine metal particles, so that super-oxide anions are easily produced.
Metals which can be used alone as catalysts are desirable, and Au, Pt, Ag, Pd and Rh are especially desirable from the standpoint of safety, since such elements have been historically demonstrated to be safe even in the case of long-term used. From the standpoint of stability as metals, Au, Pt and Pd are more preferable.
The feature of the present invention is that the present invention has established a method for supporting and forming ultra-fine metal particles on the surfaces of a photocatalytic substance consisting of fine particles. In conventional manufacturing methods, it is possible to support micron-sized fine metal particles; however, such methods are incapable of forming and supporting nano-scale ultra-fine metal particles. The limitations of such conventional manufacturing methods also hinders the improvement of the photocatalytic efficiency.
Conventional manufacturing methods have used metal salts or powdered metals as raw materials. In the present invention, on the other hand, organo-metallic compounds which can be reduced by heating are used. As a result, a great improvement in photocatalytic efficiency is achieved. The term xe2x80x9creducible by heatingxe2x80x9d means that the metal alone can be isolated from the organo-metallic compound when this compound is heated, i.e., that the other organic components can be separated out. Among such organo-metallic compounds, organo-metallic complexes are especially suited to the purpose of the present invention. However, it goes without saying that there are no particular restrictions on the compounds used, as long as these compounds are organo-metallic compounds which can be reduced by heating.
Examples of such compounds include Li compounds such as ethyllithium and p-dimethylaminophenyllithium, etc.; Na compounds such as n-propylsodium and 2-methylfurylsodium, etc.; K compounds such as ethylpotassium and phenylpotassium, etc.; Rb compounds such as ethylrubidium and triphenylmethylrubidium, etc.; Cs compounds such as ethylcesium benzylcesium, etc.; Be compounds such as dimethylberyllium and isopropylberyllium methoxide, etc.; Mg compounds such as methylmagnesium iodide and dimethylmagnesium, etc.; Ca compounds such as dimethylcalcium and phenylcalcium iodide, etc.; Sr compounds such as ethylstrontium iodide and dimethylstrontium, etc.; Ba compounds such as dimethylbarium and phenylbarium iodide, etc.; Zn compounds such as dimethylzinc and diethylzinc isoquinolinate, etc.; Cd compounds such as diisobutylcadmium and diphenylcadmium, etc.; Hg compounds such as methylmercury bromide, methylmercury iodide and bis(trifluoromethyl), etc.; As compounds such as dimethylarsenic and phenyldichloroarsenic, etc.; Sb compounds such as dimethylbromoantimony and trimethylantimony, etc.; Bi compounds such as dimethylbismuth and trimethylbismuth, etc.; Se compounds such as methyl selenocyanate and dimethyl selenide, etc.; Te compounds such as dimethyl telluride and xcex2-dimethyl telluride dichloride, etc.; Po compounds such as polonium carbonyl and dimethylpolonium, etc.; B compounds such as tricyclooctylborane and 2,4-dimethylborane, etc.; Al compounds such as trimethylaluminum and dimethylaminodimethylaluminum, etc.; Ga compounds such as trimethylgallium and phenyldibromogallium, etc.; In compounds such as trimethylindium and diphenylbromoindium, etc.; Tl compounds such as dimethylbromothallium and dimethylmethoxythallium, etc.; Cu compounds such as copper tricarbonyl, phenylcopper and bis(chlorocopper)acetylene, etc.; Ag compounds such as isobutenylsilver and phenylsilver, etc.; Au compounds such as methyldibromogold, trimethylgold and diisopropylcyanogold, etc.; Pd compounds such as dichloro-(cyclooctadiene-1,5)-palladium and xcfx80-cyclopentadienyl-xcfx80-cyclopentenylpalladium, etc.; Pt compounds such as xcfx80-cyclopentadienyl-xcfx80-allyl-platinum, dychloro-(syclooctadiene-1,5)-palladium etc.; Re compounds such as methyl-penta(carbonyl)-rhenium and chloro-bis(phenylacetylene)-rhenium, etc.; Rh compounds such as xcfx80-cyclopentadienyl-di(ethylene)-rhodium and octa(carbonyl)-rhodium, etc.; Ru compounds such as penta(carbonyl)-ruthenium and xcfx80-cyclopentadienyl-methyl-di(carbonyl)-ruthenium, etc.; Tc compounds such as cyclopentadienyl-tri(carbonyl)technetium, etc.; Ti compounds such as methyl-trichloro-titanium, di-xcfx80-cyclopentadienyltitanium and triisopropoxy-phenyl-titanium, etc.; V compounds such as hexa(carbonyl)-vanadium and di-xcfx80-cyclopentadienyl-dichloro-vanadium, etc.; W compounds such as hexa(carbonyl)tungsten and tri(carbonyl)-(benzene)-tungsten, etc.; Zr compounds such as cyclopentadienyltrichlorozirconium, etc.; Co compounds such as xcfx80-allyl-tri(carbonyl)cobalt and di-xcfx80-cyclopentadienylcobalt, etc.; Cr compounds such as xcfx80-cyclopentadienyl-chloro-di(nitrosyl)chromium, tri(carbonyl)-(thiophene)chromium and dibenzenechromium, etc.; Fe compounds such as dibromotetra(carbonyl)iron and tetra(carbonyl)-(acrylonitrile)iron, etc.; Ir compounds such as tri(carbonyl)-iridium, etc.; Mn compounds such as bromopenta(carbonyl)manganese, etc.; Mo compounds such as tri(carbonyl)-(benzene)-molybdenum, etc.; Ni compounds such as tetratri(carbonyl)nickel and diacrylonitrilenickel, etc.; Os compounds such as (benzene)-(cyclohexadiene-1,3)osmium, etc.; Si compounds such as methyltrichlorosilane and methyldifluorosilane, etc.; Ge compounds such as hexaethyldigermanium and allylgermanium trichloride, etc.; Sn compounds such as ethyltin trichloride and (2-cyano-1-methylethyl)triphenyltin, etc.; and Pb compounds such as triethyl-n-propyllead and triethyllead methoxide, etc.
From the standpoints of stability and safety as metals, as described above, it is especially desirable to use at least one Au compound, Ag compound, Pd compound, Rh compound or Pt compound. Even more desirable are compounds containing Au, Ag, Pd, Rh or Pt and a sulfur-containing organic component, and most desirable are compounds containing Au, Pd, Rh or Pt and a sulfur-containing organic component. Examples include alkylmercaptans such as methylmercaptan, ethylmercaptan, propylmercaptan, butylmercaptan, octylmercaptan, dodecylmercaptan, hexadecylmercaptan and octadecylmercaptan, etc.; thioglycolic acids such as butyl thioglycolate, etc.; and other compounds such as trimethylolpropane tris-thioglycolate, thioglycerol, thioacetic acid, thiobenzoic acid, thioglycol, thiodipropionic acid, thiourea, t-butylphenylmercaptan and t-butylbenzylmercaptan, etc. In addition, balsam gold (C10H18SAuCl1-3), balsam platinum (C10H18SPtCl1-3), balsam palladium (C10H18SPdCl1-3) and balsam rhodium (C10H18SRhCl1-3), etc., may also be used.
When one of the above-described organo-metallic compounds and a powdered photocatalytic substance such as titanium dioxide, etc., are dispersed in an appropriate universally known hydrophilic solvent, a hydrophobic colloid of the organo-metallic compound is formed; and numerous particles of this organo-metallic compound colloid adhere to the surfaces of the photocatalytic powder particles. If this mixed solution is dried, and the remaining solid residue is fired, the organic component is driven out of the organo-metallic compound, so that only the metal is supported on the surfaces of the fine photocatalyst particles in the form of nano-scale ultra-fine particles. Such a procedure includes cases where drying and firing are performed as a continuous process, e.g., cases where the solvent is evaporated by heating the mixed solution, after which the solid residue is fired by further heating, etc.
Furthermore, as a different manufacturing method, if a colloidal solution of one of the above-described organo-metallic compounds and a powdered photocatalyst are sprayed toward each other so that numerous particles of the colloid are caused to adhere to the surfaces of the photocatalyst powder particles, and the photocatalyst powder particles with these adhering colloid particles are then subjected to a heat treatment while falling downward, and fine photocatalyst particles supporting ultra-fine metal particles can be continuously manufactured.
The solution concentration of the organo-metallic compound used may be appropriately set in accordance with the final product, etc., and ordinarily, this concentration is set at 0.1 wt % or greater, and preferably at 0.5 to 50 wt %. The solvent may be appropriately selected in accordance with the type of organo-metallic compound used; presently known organic solvents such as alcohols, esters and aromatic solvents, etc., may be used.
Known additives such as co-catalysts, etc., may also be contained in the above-described mixed solution in amounts which cause no loss of the effect of the present invention. Examples of co-catalysts which can be used include V, Mo, W, Nb, Cr and Ta, etc., or oxides of these metals, as well as alkali metals (Li, Na, K, Rb, Cs and Fr), alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra), and other heavy alkali metals.
The firing temperature used in the present invention is ordinarily a temperature above the reduction deposition temperature of the organo-metallic compound used, and may be appropriately varied within the temperature range below the melting point of the metal deposited by reduction. This may be described more concretely as follows: i.e., in order to isolate the metal from an organo-metallic compound such as an organo-metallic complex, it is necessary to decompose the organo-metallic compound completely, and to remove the organic atoms so that only the metal atoms remain. The temperature at which this is accomplished is defined as the reduction deposition temperature of the metal. Next, it is necessary to cause the aggregation of the isolated metal atoms, and to rearrange these atoms into ultra-fine metal particles. In regard to the upper-limit temperature of this process, any temperature below the melting point of the bulk metal is permissible; it is desirable that this temperature be 80% of the melting point of the deposited metal or lower, preferably 70% of such a melting point or lower. Furthermore, the firing atmosphere may be an oxidizing atmosphere or a thin air atmosphere; this atmosphere may be appropriately selected in accordance with the final product.
Furthermore, a major special feature of the present invention is that the above-described photocatalyst supporting ultra-fine metal particles, which has an extremely superior photocatalytic capacity, is supported on a base material. In cases where titanium dioxide alone is supported on a base material, the limits on the decomposing power of the titanium dioxide translate into limits on the self-cleansing decomposing power of the base material, so that there are limits on the quality of the base material as a commercial product. In the present invention, the quality of the above-described base material as a commercial product is greatly improved by causing a photocatalyst supporting ultra-fine metal particles to be held on said base material. The daily living environment is full of environmental contaminants and substances harmful to the human body; and merely by installing the base material of the present invention in such a space, the self-cleansing decomposing function of the base material is caused to act so that the above-described organic substances are decomposed into water and carbon dioxide, thus insuring a cleansing of the environment. The decomposition of inorganic substances is more difficult than the decomposition of organic substances; however, the decomposition of organic substances eliminates the adhesion to the base material of inorganic substances such as dust, etc., which use organic substances as a binder, so that long-term cleansing of the base material can be achieved.
Base materials used in the present invention may be classified into one-dimensional base materials, two-dimensional base materials and three-dimensional base materials. Fibers are a typical example of one-dimensional base materials.
Fibers include natural fibers and chemical fibers. Chemical fibers include inorganic fibers, regenerated fibers, semi-synthetic fibers and synthetic fibers. Natural fibers include animal fibers, vegetable fibers and mineral fibers (asbestos, etc.). Inorganic fibers include metal fibers, glass fibers, carbon fibers and active carbon fibers, etc. Synthetic fibers include polyester, polyacrylic, polyamide, polypropylene, polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride, polyurethane, polyalkylparaoxybenzoate and polytetrafluoroethylene fibers, etc. Furthermore, super fibers of recent years, e.g. aramide fibers, all-aromatic polyester fibers and heterocyclic fibers, etc. are also included in the fibers used in the present invention.
The above-described photocatalyst supporting ultra-fine metal particles can be held on fibers of an adsorbing material. The term xe2x80x9cfibers of an adsorbing materialxe2x80x9d refers to fibers which adsorb environmental contaminants, i.e., base materials produced by forming adsorbing materials into fibers, such as active carbon fibers, zeolite fibers, porous ceramic fibers, etc. Such materials may be produced by forming adsorbing materials into fibers, or by coating fiber-form materials with an adsorbing agent. In the following, active carbon fibers will be described as an example. In cases where fibers holding a photocatalyst supporting ultra-fine metal particles are combined with simple active carbon fibers by mix-spinning, an advantage that the active carbon fibers adsorb environmental contaminants, which can be decomposed by the fine particles of the metal-supporting photocatalyst on the other fibers, is obtainable. Furthermore, in cases where carbon fibers holding a photocatalyst supporting ultra-fine metal particles are combined with simple active carbon fibers by mix-spinning, an advantage that in addition to the above-described adsorbing and decomposing effect, the fibers have the same color so that the range of applications is expanded, can be obtained. Moreover, in cases where fine particles of a metal-supporting photocatalyst are formed on the surfaces of active carbon fibers, the fibers are superior in that such fibers used alone have an adsorbing and decomposing capacity.
Not only fibers, but also knitted fabrics formed by knitting such fibers, woven fabrics formed by weaving such fibers, and non-woven fabrics such as felts, etc. formed by molding such fibers into the form of non-woven fabrics, as well as filters, curtains, carpets and other fiber products, are also included among base materials which can be used in the present invention. Such filters can be utilized in air cleaners, water cleaners, toilet deodorizers, room deodorizers, refrigerator deodorizers, etc.
Examples of planar two-dimensional base materials which can be used in the present invention include window glass, mirrors, tables, wall materials, tiles, sliding door screens, coverlets, etc. Examples of solid three-dimensional base materials which can be used include toilets, furniture, ornamental items, etc. By providing the photocatalyst supporting ultra-fine metal particles of the present invention on the surfaces of such base materials, organic environmental contaminants, substances harmful to the human body and unpleasant odors, etc. can be naturally cleansed and decomposed by means of ultraviolet light from natural sunlight, fluorescent lamps or ultraviolet lamps. Since organic matter is decomposed, adhering inorganic contaminants using organic matter as a binder also become less likely to adhere.
Various methods can be used so as to hold the above-described photocatalyst supporting ultra-fine metal particles on the base material. For example, such methods include: an immersion method in which a powder consisting of a photocatalyst supporting ultra-fine metal particles is dispersed in an appropriate solvent, and the base material is immersed in this solvent, thus applying fine particles of the metal-supporting photocatalyst to the base material; a spray method in which a solvent containing a dispersed photocatalyst supporting ultra-fine metal particles is sprayed onto the base material, and a coating method that uses rollers or brushes in the case of two-dimensional base materials or three-dimensional base materials.
Furthermore, there are also methods in which the photocatalyst supporting ultra-fine metal particles is electrostatically adsorbed on the base material. The fine particles of a photocatalyst supporting ultra-fine metal particles and base material are charged with static electricity in a natural state; accordingly, a powder of the fine particles of a photocatalyst supporting ultra-fine metal particles can be sprayed onto the base material by means of the above-described electrostatic adsorbing force so that the photocatalyst is applied thereon, or the base material can be pushed into a powder of the photocatalyst so that the photocatalyst is likewise applied thereon. Furthermore, using the principle of electrical dust collection, it would also be possible first to forcibly charge the fine particles of a photocatalyst supporting ultra-fine metal particles by means of a corona discharge, and then to apply the particles to the surface of the base material located between the electrode plates, on one of the electrode plates or outside the electrode plates by means of the electric field force generated between the electrode plates.
By heating the base material, to which the fine particles of a photocatalyst supporting ultra-fine metal particles have been applied by one of the above-described methods, to an appropriate temperature, it is possible to hold the photocatalyst particles firmly on the base material. In an even more effective method, the photocatalyst supporting ultra-fine metal particles can be firmly held on the base material by coating the surface of the base material beforehand with a binder which can be cured by heating, applying the photocatalyst supporting ultra-fine metal particles on top of said binder by means of one of the above-described methods, and then curing the binder by heating.
Furthermore, the above-described photocatalyst supporting ultra-fine metal particles can be held on the surfaces of carbon fibers or active carbon fibers by the following manner: the fine particles of photocatalyst supporting ultra-fine metal particles are applied by one of the above-described methods to the surfaces of carbon fibers or active carbon fibers prepared beforehand as a base material, and then the fibers are fired at a prescribed temperature. As a result, the photocatalyst supporting ultra-fine metal particles can be firmly held on the carbon fibers or active carbon fibers.
Furthermore, it would also be possible to convert fiber raw materials into carbon fibers by means of a carbonization treatment or to further convert such fibers into active carbon fibers by means of an activation treatment, and then to cause the photocatalyst supporting ultra-fine metal particles to be held on the resulting carbon fibers or active carbon fibers by one of the methods described above. Fiber raw materials are not limited to simple fibers alone and include fabrics such as knitted or woven fabrics, etc., and other molded fiber products.
Carbon fiber raw materials include rayon fibers, pitch fibers produced by melt-spinning petroleum pitch or coal pitch, acrylic fibers and many other types of fibers; and more or less the same firing method can be used in order to convert these fibers into carbon fibers or active carbon fibers. In particular, the main raw material for carbon fibers is PAN (polyacrylonitrile), and fibers obtained by spinning this polymer are acrylic fibers. As will be described below for acrylic fibers, such acrylic fibers are converted into carbon fibers when heated at a temperature of 1,000 to 1,800xc2x0 C. in an inert atmosphere.
When such carbon fibers are subjected to an activation treatment in a mixed gas consisting of steam, carbon dioxide, nitrogen, etc., active carbon fibers in which countless micropores are formed can be produced; and the photocatalyst supporting ultra-fine metal particles as described above is held on such active carbon fibers. Because of the adsorbing power of these active carbon fibers and the decomposing power of the photocatalyst supporting ultra-fine metal particles, the self-cleansing decomposing capacity can be greatly improved.
Active carbon fibers that hold photocatalyst supporting ultra-fine metal particles can also be manufactured by means of a continuous treatment from ultra-fine metal particles, fine photocatalyst particles and a fiber raw material such as an acrylic material, etc. More specifically, by running the fiber raw material through a heating furnace and performing a carbonization treatment and an activation treatment, active carbon fibers can be manufactured. In a reaction column, a colloid of an organo-metallic complex and fine particles of a photocatalyst are sprayed toward each other, thus producing fine photocatalyst particles with an adhering colloid in space; and when the active carbon fibers are run through an intermediate point in the falling path of the fine particles, the fine photocatalyst particles with the adhering colloid are applied to the surfaces of the active carbon fibers. If these active carbon fibers are run through a heating furnace at approximately 500xc2x0 C., active carbon fibers on which the above-described photocatalyst supporting ultra-fine metal particles is firmly held can be continuously manufactured.